Sewing threads are described for example by E Oxtoby in Spun Yarn Technology, Butterworth & Co. (1987), at pages 214-215. Sewing threads may be of continuous filament yarn or of staple fibre. For cellulosic articles, threads which comprise cotton (often mercerised cotton) are commonly used. It is often desirable that the sewing thread and the article to be sewn should be of the same or similar materials for compatibility of properties. Thus, if a sewn article is to be subsequently dyed, it is desirable that the article and the sewing thread should exhibit similar dyeing characteristics.
The tensile strength of cotton sewing threads is often not as high as could be desired. This is true even of mercerised cotton threads, which are of higher strength than unmercerised cotton threads. Accordingly, it is known to make sewing threads from a blend of cotton and a synthetic fibre such as polyester or polyamide which is stronger than cotton. Such threads generally comprise more than 30 percent by weight of the synthetic fibre, because it is known that the strength of such sewing thread which comprises less than about 30 percent by weight of the synthetic fibre may be unsatisfactorily low, even to the extent of being lower than that of a thread consisting solely of cotton. The inclusion of synthetic fibre also serves usefully to increase the abrasion resistance of the thread. Good abrasion resistance is a desirable feature in a sewing thread, because the sewn thread is often exposed at the surface of a sewn article, thus rendering it especially liable to abrasion. However, such blended sewing threads in general exhibit different dyeing characteristics from those of the bulk of a sewn article, for example a textile article such as a woven or knitted garment which comprises a major proportion of cellulosic fibre.